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Ultrafast Photocarrier Dynamics Related to Defect States of Si1_xGex Nanowires Measured by Optical Pump-THz Probe Spectroscopy
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- Authors
- Jung Min Bae, Woo-Jung Lee, Seonghoon Jung, Jin Won Ma, Kwang-Sik Jeong, Seung Hoon Oh, Seongsin M. Kim, Dongchan Suh, Woobin Song, Sunjung Kim, Jaehun Park, Mann-Ho Cho
- Issue Date
- 2017-06
- Citation
- Nanoscale, v.9, no.23, pp.8015-8023
- ISSN
- 2040-3364
- Publisher
- Royal Society of Chemistry (RSC)
- Language
- English
- Type
- Journal Article
- Abstract
- Slightly tapered Si1-xGex nanowires (NWs) (x = 0.29-0.84) were synthesized via a vapor-liquid-solid procedure using Au as a catalyst. We measured the optically excited carrier dynamics of Si1-xGex NWs as a function of Ge content using optical pump-THz probe spectroscopy. The measured -?봗/T0 signals of Si1-xGex NWs were converted into conductivity in the THz region. We developed a fitting formula to apply to indirect semiconductors such as Si1-xGex, which explains the temporal population of photo-excited carriers in the band structure and the relationship between the trapping time and the defect states on an ultrafast time scale. From the fitting results, we extracted intra- and inter-valley transition times and trapping times of electrons and holes of Si1-xGex NWs as a function of Ge content. On the basis of theoretical reports, we suggest a physical model to interpret the trapping times related to the species of interface defect states located at the oxide/NW: substoichiometric oxide states of Si(Ge)0+,1+,2+, but not Si(Ge)3+, could function as defect states capturing photo-excited electrons or holes and could determine the different trapping times of electrons and holes depending on negatively or neutrally charged states.